All these operations are rendered more complicated when the vessel is connected to a wellhead and is subjected to heave movement. When a tubular string connects the wellhead to the vessel, the heave movement of the vessel may stress the wellhead, the tubular string, and the equipment of the vessel connected to the tubular string. For this reasons it is known to compensate the heave movement of the vessel to minimize the above-identified stresses.
On this subject GB 2,343,466 A discloses a vessel including a main deck; and a compensation derrick mounted on the main deck. The derrick comprises a frame, a support carrier which is moveable in a direction substantially vertical with respect to the frame and is suitable to support a tubular string connected to a wellhead and a coil tubing injector, and a draw work that is connected to the support carrier and to a compensating assembly.
The vessel disclosed in GB 2,343,466 has the drawback of being dedicated to carry out coil tubing operations only and lacking flexibility.
On the contrary multi-purpose vessels operating on underwater wells need many activities to be done aboard the vessel. In particular, many heavy items are raised, positioned, lowered and assembled when suspended along the main deck.
It follows that the working conditions are rather dangerous for the operators involved in the above-identified activities aboard the vessel.